This invention relates generally to filters for the purification of liquids. In particular, the present invention relates to antimicrobial semi-permeable cast membranes such as cellulose acetate or composite polyamide, polysulfone and polyvinylidine fluoride membranes used in reverse osmosis, ultrafiltration/nanofiltration and microfiltration.
In recent years, the public has become increasingly aware of the deteriorating quality and quantity of our nation""s and the world""s fresh water supply. Pollutants, biological and toxic waste and other contaminants are being introduced into water supplies at an ever increasing rate, making such water supplies unfit for drinking and other necessary uses. For example, medical patients with low immunity are now being requested not to drink tap water, and disease and illnesses linked to poor quality drinking water have increased dramatically in recent years. This problem is especially significant outside of the United States where water quality has deteriorated to an all time low, with the major source of such contamination primarily being bacterial in nature.
In many areas of the world, potable water is not only contaminated but also scarce. In these areas, people must rely upon expensive purification systems to remove dissolved solids from seawater, brackish water, or well water. Reverse osmosis filtration systems are one of the most common solutions for improving water quality. Osmosis is the flow or diffusion that takes place through a semi-permeable membrane, such as in a living cell. The membrane typically separates either a solvent, such as water, from a solution, or a dilute solution from a concentrated solution. This membrane separation brings about conditions for equalizing the concentrations of the components on the two sides of the membrane because of the unequal rates of passage in the two directions until equilibrium is reached.
In reverse osmosis, pressure is deliberately applied to the more concentrated solution to cause the flow of solvent in the opposite direction through the membrane, for example, into the more dilute solution. In this way, the liquid can be separated from dissolved solids and thus increase the concentration of the dissolved solids in solution. Typically, the osmotic pressure of a solution containing 1000 ppm of dissolved salts is 10 psig. Most residential reverse osmosis units operate at less than 150 psig. Reverse osmosis units treating brackish waters operate at 150 to 450 psig, while those for seawater operate at 800 to 1000 psig.
The widespread use of reverse osmosis to produce potable water began in the early 1960""s when Loeb and Sourirajan developed a cast thin-skin cellulose acetate membranes for use in reverse osmosis systems. These cellulose acetate membranes provided much higher salt rejection (approaching 95%) and solvent flow than previously known reverse osmosis methods. Cellulose acetate membranes are also relatively inexpensive and are very tolerant of chlorine, which is commonly used to eliminate bacteria in water. Since the 1960""s, the use of reverse osmosis has grown dramatically in waste water applications and industrial desalinization plants to produce drinking water from brackish and sea waters. More recently, cast cellulose acetate membranes have been incorporated into consumer filtration systems to produce drinking water at the point of use. (Matsuura, T., Synthetic Membranes and Membrane Separation Processes, CRC Press, (1994)). Although cellulose acetate membranes greatly expanded the utilization of reverse osmosis treatment systems, such systems are still restricted by operational problems. For example, cellulose acetate membranes hydrolyze and biodegrade readily. Therefore, a need exists for alternative membranes for use in reverse osmosis systems.
Recently, a cast thin film composite polyamide membranes have been developed that offer better performance than cellulose acetate membranes. (See for example, U.S. Pat. Nos. 4,277,344, 4,520,044 and 4,606,943). Composite polyamide membranes have a bottom layer of reinforcing fabric usually made of polyester, on top of which is typically deposited a layer of polysulfone polymer. The layer of polysulfone polymer is typically 40 microns thick. A 0.2-micron ultrathin layer of polyamide is then cast on the top of the polysulfone layer. (Singh, R., xe2x80x9cMembranesxe2x80x9d, Ultrapure Water, March 1997). The porous polysulfone support is saturated with water-soluble amine solution, and acid chloride solution is then applied to bring about an in situ polymerization to the polyamide. For example, U.S. Pat. No. 3,551,331 describes a process for modifying the permeability of linear aliphatic polyamide membrane.
The polyamide layer enables the composite polyamide membrane to exhibit salt rejection rates greater than 99.5% at pressures much lower than the pressures used for cellulose acetate membranes. Additionally, polyamide membranes reject silica, nitrates, and organic materials much better than cellulose acetate membranes. Because of the high performance of composite polyamide membranes, these membranes are used in high purity or ultrahigh purity water systems in pharmaceutical and electronics industries. However, just as cellulose acetate membranes exhibit a limiting characteristic, for example, biodegradation, so do composite polyamide membranes. Composite polyamide membranes are also susceptible to damage from chlorine. To overcome some of these shortcomings, other types of cast membranes have been developed that use different types of polymers.
As the technology for manufacturing composite polyamide, cellulose acetate and other types of membranes has progressed, new fields of filtration, such as ultrafiltration, or nanofiltration, and microfiltration have been created. Many of these membranes utilize a support layer having a relatively high degree of porosity followed by an ultra-thin layer of another polymeric coating, such as the polyamide layer described above, that allows for a high salt rejection or rejection of various ranges of molecular weights of organic substances. Additionally, the support membranes, or structures, are either woven or nonwoven and are typically made from polyolefins, polyester, aromatic polysulfones, polyphenylenesulfones, aromatic polyether sulfone, bisphenol A, dichlorodiphenoxysulfone, aromatic polyether ketones, sulfonated polyether ketones, phenoxides made from epichlorohydrin and bisphenol A, polyvinylidene fluoride or sulfonated polyvinylidene fluoride, nylon, vinyl chloride homo- and co-polymers, polystyrene, polytetrafluorethylene, glass fiber, porous carbon, graphite, inorganic membranes based on alumina, and/or silica with coating of zirconium oxide. The support structure is either in the form of a flat sheet or a hollow fiber configuration depending on the desired characteristic nature of the final membrane.
U.S. Pat. No. 5,028,337 (xe2x80x9c""337xe2x80x9d) describes compositions of many types of cast membranes and methods of preparing the same. In particular, the ""337 patent discloses an ultra-thin polymeric coating on a porous support which may be selected from the following polymers, which may be in turn halomethylated, quaternized and/or sulfonated, as desired or necessary prior to a coating step: aromatic oxide polymers, such as 2,6 dimethyl polyphenyleneoxides, aromatic polysulfones, aromatic polyethersulfones, aromatic polyether ketones, linear polyaromatic epoxides; aryl polymers, such as polystyrene and poly (vinyl toluene) polymers; and, sulfonated poly (haloalkylene) polymers, such as sulfonated polyvinyl chloride, polyvinyl fluoride, polyvinylidene fluoride or polyvinylidene fluoride/hexafluoropropylene.
Casting of polymeric membranes on the support structures described above that are made of polysulfones, polyether sulfones, polyether ketones, polyvinylidene fluoride, sulfonated polyvinylidene fluoride or polyacrylonitrile may be accomplished by any number of casting procedures extensively described in published patent and technical literature. (See, for example, U.S. Pat. Nos. 3,556,305; 3,567,810; 3,615,024; 4,029,582; and 4,188,354; GB 2,000,720; Office of Saline Water R and D Progress Report No. 357, October 1967; Reverse Osmosis and Synthetic Membranes, Ed. Sourirajan; Murari et al; J. Member Sci. 16:121-135 and 181-193 (1983)).
Typically, in the manufacture of cast membranes on a support structure, the polymer or its derivatives are dissolved in a suitable solvent, such as N-methyl-pyrollidone (xe2x80x9cNMPxe2x80x9d), di-methyl formamide (xe2x80x9cDMFxe2x80x9d), di-methyl sulfoxide, hexamethylphosphoramide, N,N-dimethylacetamide, and dioxane. Additionally, the polymer or its derivatives are dissolved in a solvent mixture of the aforementioned solvents with or without cosolvents, partial solvents, nonsolvents, salts, surfactants or electrolytes for altering or modifying the membrane porosity, flux and rejection properties, such as acetone, methanol, formamide, water, methyl ethyl ketone, triethyl phosphate, sulfuric acid, HCl, partial esters of fatty acids and sugar alcohol, or their ethylene oxide adducts, sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, NaOH, KCl, zinc chloride, calcium chloride, lithium nitrates, LiCl, and magnesium perchlorate. The concentration of polymer in the casting solution is dependent on its molecular weight and also on the additives that may be present. The widest range of concentration may be between 5% to 80%, but more typically, the range of concentration is from 15% to 30%. The casting temperature may vary from xe2x88x9220xc2x0 C., to 100xc2x0 C. and is typically from 0xc2x0 C. to 10xc2x0 C.
The casting solution can be applied to porous supports by any conventional techniques that are familiar to those skilled in the art. The wet film thickness is typically between 100 to 500 microns for the flat membrane, but the broadest range may be between 15 micron to 5 mm. For hollow fibers or tubular forms, the thickness can be even higher. In order to control the porosity of the cast film, the wet film on the support may be immersed in a precipitating bath immediately or may be subjected to partial drying for 5 seconds to about 48 hours under ambient conditions or elevated temperatures, under atmospheric conditions or under vacuum. The precipitating bath is usually made up of water to which small amounts of solvent, such as DMF or NMP, and surfactant, such as sodium dodecyl sulfate, are added. The bath is usually maintained at a temperature of between 0xc2x0 C. to 70xc2x0 C. A typical precipitating bath is water with 0.5% sodium dodecyl sulfate at a temperature of 4xc2x0 C.
Some membranes are formed with a polymer solution containing a component, which may be leached out in water or other solvent. The cast membrane is dried and then a subsequent immersion step removes the leachable material to result in the creation of porosity. Other membranes, such as cellulose acetate, are cast from a polymer solution without any leachable materials, dried and annealed by subjecting the material to heat or pressure to further modify the pore structure.
Another process for making membranes used for ultrafiltration and microfiltration involves extrusion and controlled thermostretching and cooling. Examples of materials used in such membranes include microporous polytetrafluoroethylene, polypropylene and polyethylene. Chemical compounds of various sizes and molecular weights are selectively filtered out by manipulating the pore sizes of these membranes. For example, membranes used in reverse osmosis, or hyperfiltration, remove particles of 1 to 10 Angstrom units and include chemical compounds of about 180 to 15,000 molecular weights. Ultrafiltration filters particles of 30 to 1100 Angstrom units that include macromolecules of molecular weight of 10,000 to 250,000. Microfiltration, which is mainly used to remove bacteria from a solution, filters in the range of 500 Angstroms to 20,000 Angstroms (0.05 to 2 micron) (Lonsdale, H. K. xe2x80x9cThe Growth of Membrane Technologyxe2x80x9d Journal of Membrane Science, 10, p.80-81 (1982)).
All of the membranes mentioned above can be in the form of a flat sheet or in a hollow fiber configuration with a bore in the center of each fiber. For example, U.S. Pat. No. 5,762,798 (xe2x80x9c""798xe2x80x9d) describes a method of manufacturing a hollow fiber polysulfone membrane. Further, the membrane disclosed in the ""798 patent may be either asymmetric or symmetric. Asymmetric membranes have pore sizes on one face of the membrane that are different from those pore sizes on the other face of the membrane. Typically, the narrower pores on one of the faces give way to the tortuous branchings of the larger pores that exit on the other face. Asymmetric membranes commonly have higher fluid flux. In comparison, symmetric membranes have pore sizes that are the same on either face and have no tortuosity of pore channels.
However, the ability to remove dissolved particles from water comes with a price. Bacteria contained in the influent water are arrested by the semi-permeable membranes and, consequently, accumulate on the surface of the membranes. Bacteria typically multiply every 30 to 60 minutes and their growth is logarithmic. For example, a single bacterial cell will result in 16 million bacteria in 24 hours. The explosive growth of bacteria results in fouling of the membrane which reduces the flow of water through the membrane and can adversely affect the filtering properties of the membrane. For example, bacteria build-up typically has an adverse affect on salt rejection in a reverse osmosis membrane. (Wes Byrne, Reverse Osmosis, Chapter 9xe2x80x94Biological Fouling).
Furthermore, fouled membranes require higher operating pressures, which in turn increases operating costs. Alternatively, cleaning of reverse osmosis membranes using chemicals requires 20% of the total operating time of a reverse osmosis facility, thereby resulting in a dramatic reduction in the overall efficiency of the process. (Ebrahim, S. xe2x80x9cCleaning and Regeneration of Membranes in Desalination and Waste Water Applications: State of the Artxe2x80x9d, Proceedings of the International Desalination Association and Water Use Promotion Center World Conference, vol. 1, pp. 197-208, Yokohama, Japan (Nov. 3-6, 1993)). Standard fouling factors for reverse osmosis, ultrafiltration and microfiltration membranes are 30%, 80% and 90%, respectively. Thus, the fouling rate is the most important consideration in designing a water treatment plant that utilizes a membrane process. (Denese Tracey, xe2x80x9cMembrane Foulingxe2x80x9d, Ultrapure Water, October, 1996).
In addition to reducing water quality, fouled membranes are difficult to clean. As a result of the bacterial growth on the membrane, a gelatinous biofilm is formed on the upstream surface of the membrane, which is very difficult to remove, except through the use of strong chemical oxidants that damage the membrane. The biofilm protects the bacteria from normal cleaning and sanitizing procedures and leads to a break through of bacteria across the membrane. The bacterial penetration could also occur along defects in the membrane. Typically, bacteria are detected on the downstream side of the membrane within 48 to 72 hours. The downstream side of the membrane becomes noticeably discolored or black over time as the bacteria colonize on the downstream side of the membrane and form a biofilm. Such biological fouling can also lead to the formation of localized extremes in pH that can further damage the membrane. Thus, conventional semi-permeable filters standing alone rarely provide ultrapure (e.g. bacteria free) water. In many instances, reverse osmosis, ultrafiltration and microfiltration processes must be followed by polishing filters to clean the water of bacteria.
It should be pointed out that membranes can also be produced by processes of extrusion and thermostretching, but these suffer from the lack of homogeneity and unsuitable range of pores for this application.
What is therefore needed is a semi-permeable cast membrane filter that provides substantially ultrapure water. More particularly, a need exists for a semi-permeable membrane that may be used in reverse osmosis, ultrafiltration, nanofiltration and microfiltration to produce substantially ultrapure water without the assistance of additional filtering means. Further, a need exists for a filter membrane that resists fouling caused from bacterial growth.
The principal object of the present invention is to provide a cast semi-permeable membrane filter that provides substantially ultrapure water.
Another object of the present invention is to provide a cast semi-permeable membrane having an antimicrobial agent incorporated therein.
Another object of the present invention is to provide a cast filter membrane that achieves a high level of separation of water-soluble contaminants.
Another object of the present invention is to provide a cast membrane filter that resists fouling due to bacterial growth.
Another object of the present invention is to provide a cast membrane filter that inhibits the passage of bacteria to the downstream side of the membrane.
Another object of the present invention is to provide a cast membrane filter that reduces downtime for water treatment processes.
The present invention is a cast semi-permeable membrane filter that provides substantially ultrapure water. In particular, the present invention provides a semi-permeable membrane that may be used in reverse osmosis, ultrafiltration, nanofiltration and microfiltration to achieve a high level of separation of water soluble contaminants without the assistance of additional filtering means. The membrane comprises a non-leaching antimicrobial agent within the membrane structure.
In a most basic form, the invented cast semi-permeable membrane comprises a microporous layer of polymeric material and a non-leaching antimicrobial agent that is incorporated into the polymeric material of the microporous layer. The microporous layer may be in the form of a flat film or sheet or a hollow fiber configuration. A porous support layer, such as a reinforcing fabric, may be combined with the polymeric microporous layer that removes inorganic or bacterial contaminants. The support layer provides added mechanical strength to the semi-permeable membrane. Because of the exacting requirements with respect to pore sizes it has been found that only cast membranes are able to perform to eliminate dissolved inorganic and organic ions. No extruded membranes have been able to meet these exacting requirements so far.
In one embodiment of the present invention, the cast semi-permeable membrane comprises a thin film of cellulose acetate polymer, and the antimicrobial agent is dispersed homogeneously throughout the thin film of cellulose acetate. The antimicrobial agent is selected from the group consisting of 2,4,4xe2x80x2-trichloro-2xe2x80x2-hydroxy diphenol ether and 5-chloro-2-phenol (2,4-dichlorophenoxy). Preferably, the antimicrobial agent is present in a concentration between about 2,500 ppm and 20,000 ppm by weight based upon a total cellulose acetate polymer content of about 15 tol 8%.
In another embodiment of the claimed invention, the polymeric material of the cast semi-permeable membrane comprises a microporous polysulfone material having a thickness between about 20 microns and 60 microns and the antimicrobial agent dispersed homogeneously throughout the microporous polysulfone material. The antimicrobial agent is selected from the group consisting of 2,4,4xe2x80x2-trichloro-2xe2x80x2-hydroxy diphenol ether and 5-chloro-2-phenol (2,4-dichlorophenoxy). The antimicrobial agent is present in a concentration between about 50 ppm to 20,000 ppm by weight based upon a total polysulfone polymer content of about 15% to 18%. To form the membrane, a layer of microporous polysulfone material having a thickness of about 20 to 60 microns thick is deposited onto a reinforcing fabric that has a thickness between about 75 microns and 150 microns thick. An ultrathin layer of polyamide material between about 0.08 microns to 0.4 microns is then formed on the exposed surface of the polysulfone material by reaction of amine and acid chloride on the surface of the composite membrane in the presence of a catalyst.
In another embodiment of the present invention, the polymeric material of the cast semi-permeable membrane comprises a plurality of microporous hollow fibers made from polysulfones or polyvinylidene fluoride that incorporates the non-leaching antimicrobial agent. The antimicrobial agent is dispersed homogeneously throughout the microporous hollow fibers. These hollow fibers are preferably microporous polymeric capillary tubes having an outside diameter that is less than about 2 mm and a wall that functions as a semi-permeable membrane. The antimicrobial agent is selected from the group consisting of 2,4,4xe2x80x2-trichloro-2xe2x80x2-hydroxy diphenol ether and 5-chloro-2-phenol (2,4-dichlorophenoxy). The antimicrobial agent is present in a concentration between about 50 ppm to 20,000 ppm by weight based upon the weight of the polysulfone and polyvinylidene fluoride polymer. A wide variety of hollow fiber membranes are made depending on desired applications, including but not limited to reverse osmosis, ultrafiltration, and microfiltration.